Medical devices which incorporate inflatable or expandable balloons serve a wide variety of purposes. The balloon is carried on or affixed to a catheter shaft for delivery of the balloon to a desired location in the patient. The catheter shaft includes a lumen for introducing an inflation fluid into the balloon. For example, such catheter balloons are widely known to be useful for performing angioplasty procedures or the like, in which narrowings or obstructions in blood vessels or other body passageways are altered in order to increase blood flow through the narrow or obstructed area. More specifically, in a typical balloon angioplasty procedure, a balloon catheter is percutaneously introduced into the patient by way of the arterial system and advanced until the balloon of the catheter lies across the vascular narrowing or obstruction. The balloon is then inflated to dilate the vessel lumen at the site of the narrowing or obstruction. If desired, a stent may be positioned over the balloon and deployed at the site of the narrowing or obstruction to ensure that the dilated vessel lumen remains open. Balloon catheters find utility in a wide range of procedures, including valvuloplasty and urological procedures, among others.
The balloons of prior balloon catheters have been constructed from a wide variety of polymeric materials. These balloons each have their own advantages and drawbacks. Balloons comprising polyethylene terephthalate (PET), for example, have a relatively low degree of distention or expansion once they are inflated. This generally minimizes any potential adverse effects from overinflation or overexpansion of the balloon or any stent carried on it. Semi-distending or non-distending balloons often possess relatively high tensile strength, burst pressure and puncture resistance, qualities highly desirable for dilating tough lesions or for deploying and expanding stents carried over them.
However, body vessels such as arteries are generally tapered, and the locations at which narrowings or obstructions may occur vary, so that a balloon which closely matches the ultimately desired diameter of the vessel may not be readily available. Moreover, it may at times be desirable to be able to increase the diameter of the balloon beyond that which had been contemplated before the balloon procedure was begun. While balloons comprising materials such as polyvinyl chloride can be more distensible than PET or the like, balloons comprising such materials often possess a significantly lower tensile strength, burst pressure or puncture resistance than the less-distensible balloons. Overinflation of such balloons is also possible.
A variety of attempts have been made to construct medical device balloons from materials which yield balloons of good strength (that is, relatively high tensile strength and burst pressure, and good puncture resistance) while retaining an adequate degree of compliance, that is, an acceptable ratio of balloon diameter growth under an applied pressure to that balloon pressure. Each of these attempts possesses its own advantages and disadvantages. Balloons made from materials such as PET may possess excessive crystallinity or may be too stiff, so that such balloons may be resistant to the folding desired to minimize the profile of the catheter in which the balloon is employed; such resistance to folding is particularly problematic when the balloon is deflated following inflation during an in situ application, in order to be retracted into the distal end of the catheter for withdrawal. A minimal catheter profile is a highly desirable characteristic of balloon catheters, however. Some materials do not readily accept coating with drugs or lubricants, and some materials are difficult to fuse or adhere to conventional catheter shafts. Balloons made of some biaxially oriented nylons or polyamides have been asserted to overcome some of these problems.
Catheter balloons comprised of block copolymers have been suggested as a way of achieving an acceptable combination of balloon strength and elasticity. For example, it is known that catheter balloons can be constructed from polyamide/polyether block copolymers, commonly identified by the acronym PEBA (polyether block amide). Many of such copolymers can be characterized by a two phase structure, one being a thermoplastic region that is primarily a polyamide, semicrystalline at room temperature, and the other being an elastomer region that is rich in polyether. Balloons comprising such copolymers are asserted to possess a desirable combination of strength, compliance and softness. Catheter balloons comprising blends of two or more such copolymers are also known, and it has been asserted that irradiating such blends can enhance the properties of the resulting balloons, including increased burst pressures.
It would be highly advantageous to have medical devices which included expandable or inflatable balloons with improved strength, for example, with greater tensile strength, burst pressure and/or puncture resistance, while simultaneously possessing acceptable compliance (in this case, an acceptable ratio of balloon diameter growth to balloon pressure). It would also be highly advantageous to have medical devices made from materials which meet a variety of desirable processing criteria, including thermal stability, non-toxicity, non-volatility, high boiling point (preferably, solid at room temperature), high flash point, insensitivity to moisture and commercial availability.
It would also be advantageous to have balloon-type or other medical devices (such as catheters) which had a varying durometer or durometer hardness, that is, a varying resistance to deformation upon the application of a transverse force, but which did not need to be constructed from multiple pieces of different durometers. “Durometer” or “durometer hardness” usually refers to the resistance of materials such as rubber or plastics to deformation, typically to deformation by an indenter of specific size and shape under a known load. The stiffness or resistance to lateral deformation of an elongate rubber, plastic or portion of a medical device often correlates to durometer hardness, as does balloon burst pressure. The stiffness or resistance to lateral deformation of such an elongate portion also often correlates to the modulus of elasticity or flexural modulus of the rubber, plastic or other material of which the elongate portion is made. For brevity, the use of the phrase “varying durometer” herein refers to changes in any or all of durometer hardness, stiffness, resistance to lateral deformation, modulus of elasticity, flexural modulus or other desirable functional property. Use of the word “durometer” herein is therefore not limited to durometer hardness or to properties which correlate to durometer hardness. As used herein, “durometer” instead also includes properties such as modulus of elasticity and flexural modulus which do not necessarily correlate to durometer hardness, since materials having the same durometer hardness may have different moduli of elasticity or different flexural moduli, and thus different stiffnesses.
Varying durometer along the length of a medical device enables different parts of a device to perform different functions. Unfortunately, present methods or structures for achieving a variable durometer along the length of a catheter shaft or other medical device entail securing two or more separate pieces of different durometer by adhesion, heat bonding, butt bonding, sonic welding, mechanical means or the like. The resulting structures have a very rapid or abrupt change in composition, and therefore a very rapid or abrupt change in durometer, at the junction of the pieces of different durometer. One drawback of such structures is that the very rapid or abrupt change in durometer creates a kink point at which the catheter shaft or the like is subject to folding over during use, making the catheter shaft or the like more difficult to advance in the patient. Eliminating this abrupt change in composition while allowing the medical device (or portion thereof) to have different durometers along its length would make it easier to advance a catheter shaft or the like in a patient. Moreover, particularly in devices having very small cross-sectional diameters, it is often difficult to reliably secure together different pieces of very small diameter. This difficulty would be avoided if the different pieces of the medical devices could be continuously formed. It would further be advantageous to achieve a varying durometer without the need for heat bonding two or more separate pieces, as heat bonding the pieces adds heat history to them, along with an associated risk of degradation at the bond site. It would also be advantageous to have medical devices in which a change in durometer was gradual over an appreciable length of the devices, that is, over a length long enough to improve the practical utility of such devices, such as by obviating kinking or the like.